Limescale problem with shower unit

[TVR keith]

Son in law fitted a Triton shower, (cold inlet only, no pump just heats water to flexi pipe). Unit is connected to rising main to cold tank in loft. It is a hard water area
15 Months later pressure relief valve has blown due to blockage in flexi pipe/shower head.
Fitted new valve and run shower without shower head. Loads of limescale keeps coming out.
He will clean shower head with descaler & check all holes are clear. No scale comes out of kitchen tap, no work recently done to plumbing in house, no roadworks , next door have no problem.
Obviously if more limescale comes down the flexi pipe the head will get blocked and blow the pressure relief valve again .
Any ideas about where the limescale is coming from and how to stop it?

[Dogwatch]

It's the heater unit in the shower descaling itself. Not much you can do except to reflect that each lump of loosened scale is improving the efficiency of the heater!

The fine holes in the shower head will always be stopping the loosened scale from getting out though. Sorry!

[DrDeAtH]

fit a limefighter into the supply pipe for the shower that should help a little.